Hexavalent chromium (Cr(VI)) and arsenic are both major environmental concern, because they are known human lung carcinogens and exposure is widespread. It is likely that humans with co- exposure to Cr(VI) and arsenic could have even greater risks of lung cancers. The proposed research focuses on mechanisms of metal-induced carcinogenesis and investigates the hypothesis that arsenic synergistically increases Cr(VI) carcinogenicity as a consequence of interference with the repair of DNA double strand breaks. Our data show that arsenic synergistically increases Cr(VI)-induced chromosome damage and DNA double strand breaks in human lung cells. Arsenic inhibits the repair of this DNA damage and the expression of Mre11, a protein which plays an essential role in the repair of double strand breaks. Our studies also show that Mre11 deficiency leads to Cr(VI)-induced neoplastic transformation. We will test our hypothesis through three interrelated specific aims: 1) characterize the effect of arsenic on Cr(VI)-induced chromosome aberration, DNA double strand breaks and their repair; 2) determine how arsenic inhibits DNA double strand break repair; 3) determine if arsenic-induced modulation of MRN contribute to Cr(VI) and arsenic-induced neoplastic transformation. These three aims will use a combination of established and state-of-the-art toxicological, cytogenetic, and molecular biological techniques and with the following approach: 1) Cr(VI)-induced chromosome damage and DNA double strand breaks will be measured by cytogenetic assay and comet assay as well as the production of gamma-H2A.X foci, respectively; 2) Gene expression, immune-localization, protein expression studies and gene silencing will determine the mechanisms of arsenic synergistic effect; and 3) DNA transfection and transformation assays will determine how double strand break repair protects cells from arsenic and Cr(VI)-induced neoplastic transformation. This is the only study investigating the carcinogenicity of co-exposure of Cr(VI) and arsenic, which is a critical area of study as people are exposed to these metals simultaneously under most conditions. Our results will lead to the first reports of detailed information on the effect of arsenic on DNA double stran break repair machinery and the first identification of DNA double strand break repair proteins targeted by arsenic. This research is significant because it will provide: 1) An understanding of Cr(VI) and arsenic's carcinogenic mechanism; 2) Essential information to better assess the risk of exposure to these metals; and 3) A mechanistic approach for further study of arsenic, Cr(VI), other metals, and lung cancer in general. PUBLIC HEALTH RELEVANCE: Both Hexavalent chromium (Cr(VI)) and arsenic are major environmental hazards and are associated with human lung cancer. Despite that people are exposed to these metals simultaneously under most conditions, little is known about the potential co- exposure impact. This research will investigate how arsenic disrupts DNA damage repair which results in enhancement of Cr(VI)-induced lung cancer.